Enhanced Metakaolin Reactivity in Blended Cement with Additional Calcium Hydroxide

This study aims to increase the pozzolanic reactivity of metakaolin (MK) in Portland cement (PC) blends by adding additional calcium hydroxide (CH_add) to the initial mixture. Cement paste samples were prepared with PC, MK and water with a water-to-binder ratio of 0.6. Cement replacement ratios were chosen from 5 to 40 wt.% MK. For higher replacement ratios, i.e., 20, 30 and 40 wt.% MK, CH_add was included in the mixture. CH_add-to-MK ratios of 0.1, 0.25 and 0.5 were investigated. Thermogravimetric analysis (TGA) was carried out to study the pozzolanic reactivity after 1, 7, 28 and 56 days of hydration. A modified mass balance approach was used to normalize thermogravimetric data and to calculate the calcium hydroxide (CH) consumption of samples with CH_add. Results showed that, without CH_add, a replacement ratio of 30 wt.% or higher results in the complete consumption of CH after 28 days at the latest. In these samples, the pozzolanic reaction of MK turned out to be restricted by the amount of CH available from the cement hydration. The increased amount of CH in the samples with CH_add resulted in an enhanced pozzolanic reaction of MK as confirmed by CH consumption measurements from TGA.

Influence of different calcined clays to the water transport performance of concretes

This study compares the pozzolanic activity of kaolinitic and illite clays after calcination as they are the most abundant types of clays worldwide. The impact of calcined clays on microstructure development is tested by determining the porosity of cement paste as well as by phase and optical analysis of cement matrix and interfacial transition zone of concrete. As water transportation with damaging carbonate or chloride ions is the main reason for reduced durability of reinforced concrete, the results are complemented with water absorption tests on blended concretes. At 28 days, system with calcined kaolinitic clay reveal a higher densification of its microstructure with lower water absorption rate compared to plain concrete and concrete with calcined illite clay, which is related to its higher pozzolanic reactivity. Nonetheless calcined illite clays can be considered as low-cost clinker replacement as a significant pozzolanic contribution is detectable and after 90 days, the water absorption behaviour is comparable with those of the other systems investigated.

The Pozzolanic Activity of the Sediment Treated by the Flash Calcination Method

The dredged sediment has been positioned for years as alternative materials in the construction field. However, it is often necessary to apply a treatment to improve their reactivity and performance. This article aims to study the pozzolanic reactivity of fluvial sediment treated by flash calcination method at different temperatures 650 °C, 750 °C, and 800 °C. The physico-chemical, mineralogical, and environmental characteristics were studied for treated (flash-calcined sediment) and raw sediment. The pozzolanic reactivity of the flash-calcined sediments was estimated with Frattini’s test, isothermal calorimetry test, lime consumption analysis and compressive strength then compared to metakaolin which is considered as the reference. The results of the compressive strength of mortars show the detrimental effect of raw sediment on the development of resistance. Contrary to the raw sediment, the treatment of the sediments by flash calcination activates the pozzolanic reactivity of the clay phases and considerably improves the contribution of the sediments to the development of resistance and the porous structure. Moreover, the sediment calcined at 750 °C gives better properties than those obtained at 650 °C and 800 °C. The result demonstrates the feasibility of using calcined sediments as a pozzolanic mineral addition in a cementitious material.

The Solidification/Stabilization of Wastewater (From a Landfill Leachate) in Specially Designed Binders Based on Coal Ash

The aim of this study is to assess the possibility to solidify/stabilize a liquid waste from a municipal waste landfill using binders based on coal ash (fly ash and bottom ash) and specially designed cements for waste treatment (INERCEM). The leaching test proved that all cementitious systems are efficient for the solidification/stabilization of the studied wastes and can reduce the leaching potential of heavy metals present in both liquid waste and coal ash. Therefore, these wastes cease to be a source of environmental pollution. X-ray diffraction (XRD) and thermal complex analysis (DTA-TG) were used to assess the nature and amount of compounds formed in these cementitious systems during the hydration and hardening processes; ettringite, calcium silicate hydrates and CaCO3 were the main compounds formed in these systems assessed by these methods. The microstructure of hardened specimens was assessed by scanning electronic microscopy (SEM); the presence of hydrate phases, at the surface of cenospheres present in fly ash, proved the high pozzolanic reactivity of this phase.

Mechanical Transformation of Fly Ash toward Its Utilization in Cemented Paste Backfill

Fly ash (FA) showed low reactivity when being used to prepare the binder for cemented paste backfill (CPB). In the present work, wet-grinding treatment was used to increase the pozzolanic reactivity of FA and promote its sustainable utilization. The results showed that wet-grinding could be a suitable and efficient technology for FA pretreatment. Wet-grinding strongly modified the structure of FA by decreasing the crystalline phase content and the binding energy of Si 2p and Al 2p, contributing to the increase in pozzolanic reactivity of FA. The performance of CPB samples prepared by wet-ground FA was then optimized. This was reflected by the acceleration in the sample setting and increase in the strength development. The compressive strength of the CPB samples prepared by wet-ground FA for 120 min was increased by around 40% after curing for 28 d compared with the control samples.

A review of the Effect of Calcination Temperature on the Properties of Calcined Clay Concrete

Naturally occurring clays can produce an amorphous siliceous material possessing pozzolanic properties if is heated at an appropriate temperature. Calcination at the right temperature is crucial since it affects the formation of relevant phases, pozzolanic reactivity, hydration kinetics and consequently, increase the strength and durability of concrete. This paper reviews the effect of calcination temperature on the properties of mortar and concrete corporating calcined clay as partial cement replacement. It is observed that calcination temperatures close to 900°C decrease the specific surface and represent the onset for the structural reorganization of aluminosilicates. Both factors limit the pozzolanic reactivity and can consequently compromise compressive strength. The results show that mortar containing 20% calcined clay obtained compressive strength of 63MPa when calcined at 800oC, surpassing the reference cement by about 8MPa.